A zoom lens is simply an optical imaging system that changes its magnification or effective focal length while keeping the image plane stationary. Conventional technology requires that a continuous zoom lens have multiple optical elements and uses cams or gears to adjust the spacings between individual or groups of elements to vary the optical magnification. As a result, mechanical zoom lenses, such as those found on 35 mm cameras, typically take a few hundred milliseconds or more to vary magnification and are restricted to magnifying the area on-axis (i.e., the system must be directly pointed at the area to be magnified). Digital or electronic zoom, which is extremely fast and is not limited to on-axis magnification, is fundamentally different from optical zoom in that the individual pixels on the focal plane array at the image plane are simply remapped to larger areas in the display.
In FIG. 1 is shown an area-of-interest from an aerial reconnaissance image 10 that is expanded by 3× with digital zoom and with optical zoom. With digital zoom, the zoomed image 12 appears bigger, but there is no increase in information content (i.e., no increase in resolution). The individual pixels on the focal plane array are simply remapped to larger areas in the display. Thus, digital zoom is extremely fast and is not limited to on-axis magnification. Conversely, by changing the true magnification of the system, an optical zoom system actually increases the resolution over an area-of-interest, within the limits governed by diffraction and the individual pixel size on the focal plane array. In other words, when properly designed, the intrinsic amount of information over an area-of-interest can be increased in the optically zoomed image 14.
In surveillance, threat detection, and reconnaissance applications, a wide field-of-view (FOV) is often used to observe as large an area as possible, but this limits the achievable resolution over any specified area of interest of target. To achieve a higher resolution, such as required for target identification and tracking, a separate, narrow FOV is often desired. Therefore, two separate sensors are often used for acquisition, tracking, and pointing (ATP) applications: one with a low-resolution, wide FOV and the other with a narrow FOV and high resolution. Recently, pneumatically actuated systems that are very fast and dual FOV systems that rotate groups of lenses in and out of the optical path have been developed to achieve magnification. However, the size, weight, power requirements, robustness, and mechanical motion of these systems pose significant problems for space-based imaging systems. For the large entrance pupils required to achieve high resolution from space, a conventional zoom system would necessarily be extremely large and heavy.
These multiple FOV systems are also limited to on-axis magnification. Therefore, conventional, high-resolution systems must be gimbaled to point at different targets or areas of interest that are off-axis but within the field of view. The gimbals that are used to redirect the instantaneous FOV of a space-based imaging system tend to be large, heavy and require significant amounts of power, often weighing as much as the entire optical system. Depending on the size of the optics and the speed of the gimbal, they can draw hundreds or even thousands of watts of power to slew larger aperture systems. Therefore, even with state-of-the-art techniques, changing the magnification and slewing the optical system would still take hundreds of milliseconds, require significant power, and would likely induce unwanted jitter on a satellite or require momentum compensation.
Active or adaptive optics are playing an ever-increasing role in imaging and laser projection applications. Over the last 30 years, deformable mirrors (DMs) have revolutionized the imaging capability of astronomical observatories. Nearly every major observatory in the world utilizes some sort of adaptive optical system in a closed-loop architecture to compensate for aberrations caused by turbulence in the atmosphere. The success of adaptive optics in correcting atmospheric aberrations has sparked interest in the technology for other applications. For these other applications, adaptive optics offers the possibility of improving the flexibility and capabilities of imaging systems while reducing size, weight and potentially cost. In cases where closed-loop feedback is not used, the broader term “active optics” is often more appropriate.
Active optics, such as liquid crystal (LC) spatial light modulators (SLMs), can be used to adjust the diffraction-limited FOV of an imaging system very quickly without macroscopic moving parts. A wide FOV imaging system has been described that uses active optics to reduce the number of optical elements that are required and add adaptability. See D. V. Wick, et. al., “Foveated imaging demonstration,” Optics Express 10, 60–65 (2002), T. Martinez et. al., “Foveated, wide field-of-view imaging system using a liquid crystal spatial light modulator,” Optics Express 8, 555–560 (2001), U.S. Pat. No. 6,421,185 to Wick et al., and U.S. Pat. No. 6,473,241 to Wick et al. This foveated imaging system uses an active optic to selectively enhance resolution over a limited area-of-interest in a wide FOV imaging system. In this case, the multiple lenses required to minimize off-axis aberrations for a conventional wide FOV, low f/# system are replaced by a single, electrically addressed, LC SLM, minimizing the size and weight of the optical system. By using the SLM to correct aberrations at any field angle, high resolution is maintained over a limited area with lower resolution in peripheral areas, similar to the operation of the human eye. Furthermore, the area-of-interest can be moved anywhere within the FOV of the system on a millisecond time scale. In addition, the variable resolution in the image lends itself to video compression, reducing data transmission bandwidth requirements.
Liquid crystal SLMs have also been proposed to for use in a nonmechanical zoom lens. See E. C. Tam, “Smart electro-optical zoom lens,” Optics Letters 17(5), 369 (1992). This optical zoom system uses electrically addressable continuous-phase SLMs in combination with refractive lenses to perform focusing and zooming. Tam contacted the SLM-based lens with a higher-power conventional imaging lens to provide a cascade lens with a variable focal length. Two cascade lens combinations were required to simultaneously change the effective focal length and maintain the image plane of the lens system. The zooming range could be optimized, depending on the focal length of the SLM-based lens and the overall system length. However, because Tam's SLM was in physical contact with the conventional lens, the dynamic range (i.e., zoom range) of his zoom system was limited. For example, Tam described a system in which the effective focal length changed from 9.8 cm to 14.8 cm, providing a zoom of 1.5×. Furthermore, because Tam uses SLM-based lenses to change only focus, his system is limited to zooming or magnifying on-axis, as with a conventional zoom lens. Furthermore, Tam does not use the flexibility afforded by active or adaptive optics to correct for other static or dynamic aberrations.
Accordingly, there remains a need for an active optical zoom system wherein the magnification or effective focal length of the system can be changed rapidly while keeping the image plane stationary. In particular, for space-based imaging systems used for surveillance and remote sensing, there exists a need for small, lightweight imaging sensors that are capable of quickly toggling between wide FOV for situational awareness and narrow FOV with high resolution for discrimination and identification.